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| United States Patent |
6,098,460
|
|
Otsuchi
, et al.
|
August 8, 2000
|
Acceleration sensor and shock detecting device using the same
Abstract
A small accelaration sensor which is highly sensitive over a large
frequency region and which varies little in characteristics such as
sensitivity. A piezoelectric element is formed by connecting two main
faces of retangular LiNbO.sub.3 piezoelectric substrates, in which the
polarization axes are directed oppositely. Supporters comprising
LiNbO.sub.3 directly connected to one end of the piezoelectric element.
Eletrodes of chromium-gold being 0.2 .mu.m are thick are successively
connected to the two main faces of the piozoelectric element and to the
supporters, thus to produce a cantilever structure bimorph
electromechanical transducer.
| Inventors:
|
Otsuchi; Tetsuro (Osaka, JP);
Sugimoto; Masato (Osaka, JP);
Ogura; Tetsuyoshi (Osaka, JP);
Tomita; Yoshihiro (Osaka, JP);
Kawasaki; Osamu (Kyoto, JP)
|
| Assignee:
|
Matsushita Electric Industrial Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
727302 |
| Filed:
|
October 8, 1996 |
Foreign Application Priority Data
| Oct 09, 1995[JP] | 7-261240 |
| Jul 31, 1996[JP] | 8-201616 |
| Current U.S. Class: |
73/514.34; 310/331 |
| Intern'l Class: |
G01P 015/09 |
| Field of Search: |
73/514.34,514.35,514.36,514.38,654
310/329,331
340/669
|
References Cited
U.S. Patent Documents
| 3735161 | May., 1973 | Perkins et al. | 310/329.
|
| 4812698 | Mar., 1989 | Chida et al. | 310/331.
|
| 5369057 | Nov., 1994 | Lee et al. | 73/514.
|
| 5413955 | May., 1995 | Lee et al. | 437/86.
|
| 5490422 | Feb., 1996 | Tabota et al. | 73/514.
|
| 5515725 | May., 1996 | Tabota et al. | 73/514.
|
| Foreign Patent Documents |
| 0369352 | May., 1990 | EP.
| |
| 0434878 | Jul., 1991 | EP.
| |
| 0616222 | Sep., 1994 | EP.
| |
| 654 671 | May., 1995 | EP.
| |
| 2 557 973 | Jul., 1985 | FR.
| |
| 2 685 958 | Jul., 1993 | FR.
| |
| 02248086 | Oct., 1990 | JP.
| |
| 03092764 | Apr., 1991 | JP.
| |
| 7-27784 | Jan., 1995 | JP.
| |
| 07140165 | Jun., 1995 | JP.
| |
| 07202283 | Aug., 1995 | JP.
| |
| 07260819 | Oct., 1995 | JP.
| |
Other References
Patent Abstacts of Japan, vol. 14, No. 573 (1990).
Patent Abstacts of Japan, vol. 95, No. 001 (1995).
|
Primary Examiner: Chapman; John E.
Attorney, Agent or Firm: Morrison & Foerster LLP
Claims
What is claimed is:
1. An acceleration sensor which comprises an electromechanical transducer
having a piezoelectric element formed by directly connecting two opposite
main faces of at least two piezoelectric substrates and electrodes formed
on the opposite main faces of the piezoelectric element, and supporters to
support the electromechanical transducer, wherein the main faces of the
two piezoelectric substrates are connected by bonding the atoms of the
piezoelectric substrates via at least one group selected from the group
consisting of oxygen and hydroxyl groups.
2. The acceleration sensor according to claim 1, wherein the two
piezoelectric substrates are connected to each other so that the
directions of the polarization axes are opposite.
3. The acceleration sensor according to claim 1, wherein the two
piezoelectric substrates are directly connected to each other via a buffer
layer formed on a main face of one of the piezoelectric substrates.
4. The acceleration sensor according to claim 1, wherein an end of the
electromechanical transducer is supported by a supporter.
5. The acceleration sensor according to claim 1, wherein both ends of the
electromechanical transducer are supported by supporters.
6. The acceleration sensor according to claim 1, wherein the piezoelectric
substrates comprise single crystal piezoelectric materials of 3m crystal
classes, and the angle which the main face of the piezoelectric substrates
makes with its Y axis is perpendicular to an axis of +129.degree. to
+152.degree. and includes its X axis, and a line which links center of
gravity of the piezoelectric substrates to the center of the supporting
portion is perpendicular to the X axis, where the X axis, Y axis and Z
axis are the crystal axes of the single crystal piezoelectric materials.
7. The acceleration sensor according to claim 1, wherein the piezoelectric
substrates comprise single crystal piezoelectric materials of 3m crystal
classes, and the angle which the main face of the piezoelectric substrates
makes with its Y axis is perpendicular to an axis of -26.degree. to
+26.degree. and includes its X axis, and a line which links center of
gravity of the substrates to the center of the supporting portion is
parallel to the X axis, where the X axis, Y axis and Z axis are the
crystal axes of the single crystal piezoelectric materials.
8. The acceleration sensor according to claim 1, wherein the piezoelectric
substrates comprise single crystal piezoelectric materials of single
crystal 32 crystal classes, and the main face of the piezoelectric
substrate is perpendicular to its X axis while a line which links center
of gravity of the substrates to the center of the supporting portion makes
an angle of from +52.degree. to +86.degree. with its Z axes of the
substrates, where the X axis, Y axis and Z axis are the crystal axes of
the single crystal piezoelectric materials.
9. The acceleration sensor according to claim 1, wherein the piezoelectric
substrates comprise single crystal piezoelectric materials of 32 crystal
classes, and the angle which the main face of the piezoelectric substrates
makes with its X axis is perpendicular to an axis of -26.degree. to
+26.degree. and includes its Y axis, and a line which links center of
gravity of the substrates to the center of the supporting portion is
parallel to the Y axis, where the X axis, Y axis and Z axis are the
crystal axes of the single crystal piezoelectric materials.
10. The acceleration sensor according to claim 1, wherein the piezoelectric
substrates comprise single crystal piezoelectric materials of 32 crystal
classes, and the angle which the main face of the piezoelectric substrates
makes with its X axis is perpendicular to an axis of +52.degree. to
+68.degree. and includes its Z axis, and a line which links center of
gravity of the substrates to the center of the supporting portion is
perpendicular to the Z axis, where the X axis, Y axis and Z axis are the
crystal axes of the single crystal piezoelectric materials.
11. The acceleration sensor according to claim 1, further comprising:
an amplifier circuit which converts and amplifies signals from the
acceleration sensor;
a comparator circuit which compares the signals from the amplifier circuit
with standard signals and produces an output indicative of the result of
the comparison; and
a storage device to store data regarding shock experienced by said
acceleration sensor.
12. An acceleration sensor which comprises an electromechanical transducer
having a piezoelectric element formed by directly connecting two opposite
main faces of at least two piezoelectric substrates and electrodes formed
on the two opposite main faces, and supporters to support the
electromechanical transducer, where the electromechanical transducer is
directly connected to the supporters, wherein the piezoelectric substrates
and the supporters are connected to each other by bonding the atoms
composing the substrates and supporters via at least one group selected
from the group consisting of oxygen and hydroxyl groups.
13. The acceleration sensor according to claim 12, wherein the
piezoelectric substrates comprising the electromechanical transducer and
the supporters are directly bonded via a buffer layer formed on a main
face of one of the piezoelectric substrates, on a surface of one of the
supporters, or on both of them.
14. The acceleration sensor according to claim 12, wherein the
piezoelectric substrates and the supporters are formed of the same
material.
15. The acceleration sensor according to claim 12, wherein an end of the
electromechanical transducer is supported by a supporter.
16. The acceleration sensor according to claim 12, wherein both ends of the
electromechanical transducer are supported by the supporters.
17. An acceleration sensor which comprises an electromechanical transducer
comprising a piezoelectric element formed by directly connecting two
opposite main faces of at least two piezoelectric substrates and
electrodes formed on the two opposite main faces, supporters to support
the electromechanical transducer, and packages containing the
electromechanical transducer, where the supporters are directly connected
to the packages by bonding the atoms of the piezoelectric substrates via
at least one group selected from the group consisting of oxygen and
hydroxyl groups.
18. The acceleration sensor according to claim 17, wherein the packages and
the supporters are connected by bonding the atoms composing the packages
and the supporters via a group selected from the group consisting of
oxygen and hydroxyl groups.
19. The acceleration sensor according to claim 17, wherein the packages and
the supporters are directly connected to each other via a buffer layer
formed on a surface of one of the packages, on a surface of one of the
supporters, or on both of them.
20. The acceleration sensor according to claim 17, wherein the packages and
the supporters are formed of the same material.
21. An acceleration sensor which comprises an electromechanical transducer
comprising a piezoelectric element formed by directly connecting two
opposite main faces of at least two piezoelectric substrates and
electrodes formed on the two opposite main faces, supporters to support
the electromechanical transducer, and packages containing the
electromechanical transducer, where the electromechanical transducer is
supported by directly connecting the piezoelectric substrates comprising
the piezoelectric element to the packages by bonding the atoms of the
piezoelectric substrates via at least one group selected from the group
consisting of oxygen and hydroxyl groups.
22. The acceleration sensor according to claim 21, wherein the
piezoelectric substrates and the packages are connected by bonding the
atoms composing the piezoelectric substrates and the packages via a group
selected from the group consisting of oxygen and hydroxyl groups.
23. The acceleration sensor according to claim 21, wherein the
piezoelectric substrates and the packages are directly connected via a
buffer layer formed on a main face of one of the piezoelectric substrates,
on a surface of one of the packages, or on both of them.
24. The acceleration sensor according to claim 21, wherein the
piezoelectric substrates and the packages are formed of the same material.
25. An acceleration sensor which comprises an electromechanical transducer
comprising a piezoelectric element formed by directly connecting two
opposite main faces of at least two piezoelectric substrates and
electrodes formed on the two opposite main faces, supporters to support
the electromechanical transducer, and packages comprising at least two
parts to contain the electromechanical transducer, where the two parts
comprising packages are directly connected to each other by bonding the
atoms of the piezoelectric substrates via at least one group selected from
the group consisting of oxygen and hydroxyl groups.
26. The acceleration sensor according to claim 25, wherein the parts
comprising the packages are connected to each other by bonding their atoms
via a group selected from the group consisting of oxygen and hydroxyl
groups.
27. The acceleration sensor according to claim 25, wherein the parts
comprising the packages are directly connected to each other via a buffer
layer formed on a face of a component selected from the group consisting
of the piezoelectric substrates, the supporters and the packages.
Description
FIELD OF THE INVENTION
This invention relates to an acceleration sensor used for measurement of
acceleration and for detection of vibration etc. and a method for
producing the same. More specifically, this invention relates to a small,
quality acceleration sensor and a method for producing the same. Moreover,
this invention relates to a shock detecting device using the acceleration
sensor, the output of which varies less than other sensors.
BACKGROUND OF THE INVENTION
Recently electronic devices have been more miniaturized and portable
electronics devices including note-type personal computers have been
widely used. A small, surface-mountable acceleration sensor with quality
are more needed in order to certify the reliability of such electronic
devices against shock.
A high-density hard disc can be taken as an example. If the disc is shocked
during writing operation, the head is displaced and, as a result, the data
cannot be written or the head itself can be damaged. In order to avoid
such problems, it is necessary to detect shock to the head, and then stop
writing or move the head to a safe position.
Demands for a shock detecting device acceleration sensor for an airbag
apparatus are also increased so as to protect a driver from the shock
caused by a car collision.
It is also needed to install a configuration in an apparatus which detects
shock applied to a portable device and avoids failure or malfunction of
the device due to the shock, or records the shock. Therefore, the needs
for a small acceleration sensor used for such a device have been also
increased.
Acceleration sensors employing piezoelectric materials such as
piezoelectric ceramics have been well-known. Such an acceleration sensor
can realize a high detection sensitivity by using the electromechanical
conversion characteristics of the piezoelectric materials. A piezoelectric
acceleration sensor outputs forces caused by acceleration or vibration
after converting these forces into voltage by the piezoelectric effect.
One example of such an acceleration sensor uses a cantilever structure
rectangular bimorph electromechanical transducer as disclosed in
Unexamined Japanese Patent Application (Tokkai-Hei) 2-248086. As shown in
FIG. 26 of this application, a bimorph electromechanical transducer 50
using the piezoelectric effect is produced by fastening piezoelectric
ceramics (51a, 51b) formed with electrodes (52a, 52b) with an adhesive 53
(e.g. epoxy resin). The cantilever structure shown in FIG. 27 is formed by
adhering and fixing an end of the electromechanical transducer 50 to a
fixing portion 55 with, for example, a conductive adhesive 54. Such a
cantilever structure electromechanical transducer having low resonance
frequency is used for measurement of acceleration having relatively low
frequency components. In order to measure acceleration in a high frequency
region, another type of bimorph electromechanical transducer 50, both of
whose ends are fixed to fixing portions 55 with, for example, a conductive
adhesive 54, is used (see FIG. 28). The resonance frequency can be
relatively raised by fixing both ends of the electromechanical transducer
(a structure clamped at both ends).
An acceleration sensor is formed by setting the electromechanical
transducer 50 in a package while holding the fixing portion 55 to the
inner wall of the package. Electric charge generated at the electrodes
(52a, 52b) of the electromechanical transducer 50 is conducted out to
outer electrodes via, for example, the conductive adhesive 54.
As mentioned above, adhesives including an epoxy resin are used to adhere
the piezoelectric ceramics of conventional acceleration sensors. Young's
modulus of the epoxy resin is 200.times.10.sup.-12 m.sup.2 /N, which is
bigger than that of the piezoelectric ceramic (150.times.10-.sup.12
m.sup.2 /N), so the epoxy resin absorbs the distortion of the
electromechanical transducer due to acceleration, and as a result, the
sensitivity deteriorates. In addition to that, it is still difficult to
adhere piezoelectric ceramics while keeping the thickness of the adhering
layer uniform, therefore, the characteristics of the electromechanical
transducer will vary.
The resonance frequency of a rectangular bimorph electromechanical
transducer should be stable in order to make its sensitivity stable. For
this purpose, the electromechanical transducer should be fixed firmly.
Actually, however, its metallic supporters or portions supported or fixed
by fixing portions will be displaced because of stress generated
mechanically or by temperature variation. For instance, if an
electromechanical transducer is fixed by using adhesives, the fixing
positions will change depending on the adhesive-application range, and
thus its resonance frequency will vary. In another case, the fixing
condition of the electromechanical transducer will depend on the
temperature, so the stable fixing condition is not easily maintained.
In case electromechanical transducers are respectively produced and then
set in packages, handling becomes difficult in the producing steps. As a
result, the acceleration sensor cannot be miniaturized and quantity
production becomes difficult.
The piezoelectric ceramic is produced by mixing and firing several kinds of
materials, so, its material constants vary compared to that of a single
crystal material. Therefore, sensitivity and capacitance considerably
vary.
An acceleration sensor employing piezoelectric ceramics is also used to
detect shock on a portable device. Such a device, however, considerably
varies in its sensitivity, the standard acceleration range which is set to
protect apparatuses from failure tends to be large, and, thus, precise
shock detection becomes difficult. Due to the capacitance variation, it is
difficult to design a circuit which is connected to the acceleration
sensor in order to amplify electric signals generated from acceleration,
and, thus, the amplifier degree of the circuit becomes irregular. As a
result, the output signal considerably varies, and thus the acceleration
sensor cannot reliably be used for shock detecting.
SUMMARY OF THE INVENTION
This invention aims to solve the above-mentioned problems of conventional
techniques by providing a small acceleration sensor and a method for
producing it. This acceleration sensor has high sensitivity in a large
frequency region, and its characteristics including sensitivity are
remarkably stable. Another purpose of this invention is to provide a shock
detecting device using the acceleration sensor, whose output signals are
quite stable.
In order to achieve this and other aims, a first acceleration sensor of
this invention comprises an electromechanical transducer having a
piezoelectric element formed by directly connecting at least two opposite
main faces of at least two piezoelectric substrates and electrodes formed
on the opposite main faces of the piezoelectric element, and supporters to
support the electromechanical transducer. In this first acceleration
sensor, the electromechanical transducer is constituted by directly
connecting the piezoelectric substrates without using adhesive layers like
adhesive materials. Therefore, if flexible vibration generates in the
electromechanical transducer because of acceleration, nothing absorbs the
flexible vibration. As a result, the piezoelectric substrates is stressed
without loss, a great electromotive force can be obtained, and an
acceleration sensor having high sensitivity can be provided. In addition
to that, the variations in resonance frequency and sensitivity will
considerably be reduced, since the adhesion between the piezoelectric
substrates is uniform. Furthermore, the vibration characteristics of the
electromechanical transducer do not change due to temperatures, since
adhesive layers do not exist between the piezoelectric substrates.
It is preferable in the first acceleration sensor that the main faces of
the two piezoelectric substrates are connected by bonding the atoms of the
piezoelectric substrates via at least one group selected from the group
consisting of oxygen and hydroxyl groups. In this preferred embodiment,
the main faces of the two piezoelectric substrates are directly and firmly
connected to each other at the atomic level.
It is preferable in the first acceleration sensor that the two
piezoelectric substrates are connected to each other so that the
directions of polarization axes are opposite to each other. In the
preferred embodiment, an electric charge of the same polarity is generated
on the two piezoelectric substrates even if the stresses generated in the
piezoelectric substrates are different from each other, namely,
compressive stress and tensile stress. Electromotive force is generated in
the two piezoelectric substrates in the same direction. As a result,
signals reflecting the degree of acceleration can be obtained from the
electrodes formed on both faces of the electromechanical transducer.
It is preferable in the first acceleration sensor that the two
piezoelectric substrates are directly connected via a buffer layer. In
this preferred embodiment, strong direct-connecting faces are obtained,
since the buffer layer absorbs waviness, irregularities, and foreign
materials like contaminants on the adhered faces. In addition, when a
material on which oxygen or hydroxyl groups are not easily formed by
hydrophilic treatment is used, connection via a buffer layer will provide
faces which are strongly and directly connected to each other.
It is preferable in the first acceleration sensor that an end of the
electromechanical transducer is supported by supporters. In this preferred
embodiment, an acceleration sensor having a cantilever structure can be
provided.
It is more preferable in the first acceleration sensor that both ends of
the electromechanical transducer are supported by supporters. In this
preferred embodiment, an acceleration sensor having a both-ends clamped
structure can be provided. This bothends clamped structure
electromechanical transducer enables acceleration measurement in a higher
frequency region, since the resonance frequency becomes higher compared to
the case of a cantilever structure electromechanical transducer, if the
length and thickness are common.
It is preferable in the first acceleration sensor that the piezoelectric
substrates comprise single crystal piezoelectric materials of 3m crystal
classes, and that the angle which the main face of the piezoelectric
substrates makes with the Y axis is perpendicular to an axis of
+129.degree. to +152.degree. and includes the X axis, and a line which
links the center of gravity of the substrates to the center of the
supporting portion is perpendicular to the X axis, where the X axis, Y
axis and Z axis are the crystal axes of the single crystal piezoelectric
materials. In this preferred embodiment, the piezoelectric constant of the
piezoelectric substrates is 90 to 100% of the maximum value, and thus
problems due to deterioration in sensitivity will not be found.
It is preferable in the first acceleration sensor that the piezoelectric
substrates comprise single crystal piezoelectric materials of 3m crystal
classes, and that the angle which the main face of the piezoelectric
substrates makes with the Y axis is perpendicular to an axis of
-26.degree. to +26.degree. and includes the X axis, and a line which links
the center of gravity of the substrates to the center of the supporting
portion is parallel to the X axis, where the X axis, Y axis and Z axis are
the crystal axes of the single crystal piezoelectric materials. In this
preferred embodiment, the piezoelectric constant of the piezoelectric
substrates is 90 to 100% of the maximum value, and thus problems due to
deterioration in sensitivity will not be found.
It is preferable in the first acceleration sensor that the piezoelectric
substrates comprise single crystal piezoelectric materials of single
crystal 32 crystal classes, and that the main face of the piezoelectric
substrate is perpendicular to the X axis while a line which links the
center of gravity of the substrates to the center of the supporting
portion makes an angle of from +52.degree. to +86.degree. with the Z axis,
where the X axis, Y axis and Z axis are the crystal axes of the single
crystal piezoelectric materials. In this preferred embodiment, the
piezoelectric constant of the piezoelectric substrates is 90 to 100% of
the maximum value, and thus problems due to deterioration in sensitivity
will not be found.
It is preferable in the first acceleration sensor that the piezoelectric
substrates comprise single crystal piezoelectric materials of 32 crystal
classes, and that the angle which the main face of the piezoelectric
substrates makes with the X axis is perpendicular to an axis of -260 to
+26.degree. and includes the Y axis, and a line which links the center of
gravity of the substrates to the center of the supporting portion is
parallel to the Y axis, where the X axis, Y axis and Z axis are the
crystal axes of the single crystal piezoelectric materials. In this
preferred embodiment, the piezoelectric constant of the piezoelectric
substrates is 90 to 100% of the maximum value, and thus problems due to
deterioration in sensitivity will not be found.
It is preferable in the first acceleration sensor that the piezoelectric
substrates comprise single crystal piezoelectric materials of 32 crystal
classes, and that the angle which the main face of the piezoelectric
substrates makes with the X axis is perpendicular to an axis of
+52.degree. to +68.degree. and includes the Z axis, and a line which links
the center of gravity of the substrates to the center of the supporting
portion is perpendicular to the Z axis, where the X axis, Y axis and Z
axis are the crystal axes of the single crystal piezoelectric materials.
In this preferred embodiment, the piezoelectric constant of the
piezoelectric substrates is 90 to 100% of the maximum value, and thus
problems due to deterioration in sensitivity will not be found.
A second acceleration sensor of this invention comprises an
electromechanical transducer having a piezoelectric element formed by
connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the two opposite main faces, and
supporters to support the electromechanical transducer, where the
electromechanical transducer is directly connected to the supporters. In
this second acceleration sensor, the electromechanical transducer is
produced by directly connecting the piezoelectric substrates without using
adhesive layers. Therefore, the supporting position of the
electromechanical transducer is less varied, and thus, an acceleration
sensor having less varied a resonance frequency can be provided. In
addition to that, acceleration can be transferred to the electromechanical
transducer without loss, since the electromechanical transducer is
directly connected to the supporters without using adhesives. Furthermore,
the supporting condition will not change with temperature, since adhesive
layers do not exist between the piezoelectric substrate and the
supporters.
It is preferable in the second acceleration sensor that the piezoelectric
substrates and the supporters are connected to each other by bonding the
atoms composing the substrates and supporters via at least one group
selected from the group consisting of oxygen and hydroxyl groups.
It is preferable in the second acceleration sensor that the piezoelectric
substrates composing the electromechanical transducer and the supporters
are directly bonded via a buffer layer.
It is also preferable in the second acceleration sensor that the
piezoelectric substrates and the supporters compose the same materials. In
this preferred embodiment, an acceleration sensor which is extremely
stable under temperature variation can be provided, since it is not
effected by distortion due to temperature.
It is preferable in the second acceleration sensor that an end of the
electromechanical transducer is supported by the supporters.
It is more preferable in the second acceleration sensor that both ends of
the electromechanical transducer are supported by the supporters.
A third acceleration sensor of this invention comprises an
electromechanical transducer having a piezoelectric element formed by
directly connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the two opposite main faces,
supporters to support the electromechanical transducer, and packages
containing the electromechanical transducer, where the supporters are
directly connected to the packages. In this third acceleration sensor, the
supporters of the electromechanical transducer are directly connected to
the packages without using adhesives, so the supporters are strongly
connected to the packages. Therefore, a highly sensitive acceleration
sensor in which acceleration generated on its mounting surface can be
transferred to the supporters without loss via the packages.
It is preferable in the third acceleration sensor that the packages and the
supporters are connected by bonding the atoms composing the packages and
the supporters via one group selected from the group consisting of oxygen
and hydroxyl groups.
It is preferable in the third acceleration sensor that the packages and the
supporters are directly connected via a buffer layer.
It is also preferable in the third acceleration sensor that the packages
and the supporters be comprised of the same materials.
A fourth acceleration sensor of this invention comprises an
electromechanical transducer having a piezoelectric element formed by
directly connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the two opposite main faces,
supporters to support the electromechanical transducer, and packages
containing the electromechanical transducer, where the electromechanical
transducer is supported by directly connecting the piezoelectric
substrates composing the piezoelectric element to the packages. In this
fourth acceleration sensor, the piezoelectric substrates are directly
connected to the packages without using adhesives, so the
electromechanical transducer is strongly connected to the packages. As a
result, a highly sensitive acceleration sensor can be obtained since the
acceleration the packages receive is transferred to the electromechanical
transducer without loss. In addition to that, the sections of the
acceleration sensor can be reduced since the packages function as the
supporters.
It is preferable in the fourth acceleration sensor that the piezoelectric
substrates and the packages are connected by bonding the atoms composing
the piezoelectric substrates and the packages via one group selected from
the group consisting of oxygen and hydroxyl groups.
It is preferable in the fourth acceleration sensor that the piezoelectric
substrates and the packages are directly connected via a buffer layer.
It is also preferable in the fourth acceleration sensor that the
piezoelectric substrates and the packages be comprised of the same
materials.
It is preferable in the fourth acceleration sensor that a conductive layer
is provided to the piezoelectric element excepting the electromechanical
transducer. In this preferred embodiment, outer electrodes can be formed
on the faces opposite to the supporting portions-even if the acceleration
sensor has a cantilever structure. In this preferred embodiment, it is
also possible to provide electrodes on all of the faces of the
electromechanical transducer, and thus a highly sensitive acceleration
sensor can be provided.
A fifth acceleration sensor of this invention comprises an
electromechanical transducer having a piezoelectric element formed by
directly connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the two opposite main faces,
supporters to support the electromechanical transducer, and packages being
composed of at least two parts to contain the electromechanical
transducer, where the two parts composing the packages are directly
connected to each other. In this fifth acceleration sensor, the parts
composing the packages are strongly connected to each other without using
adhesives, so the heat resistance characteristics of the connected faces
are improved. Therefore, the connecting parts do not generate gases even
if solder reflow is conducted, and thus every part composing the packages
is air-tight sealed. As a result, a reliable acceleration sensor whose
characteristics do not deteriorate can be obtained.
It is preferable in the fifth acceleration sensor that the parts composing
the packages are connected to each other by bonding their atoms via one
group selected from the group consisting of oxygen and hydroxyl groups.
It is preferable in the fifth acceleration sensor that the parts composing
the packages are directly connected to each other via a buffer layer.
In the first method of this invention for producing an acceleration sensor
comprising an electromechanical transducer having piezoelectric element
formed by connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the opposite main faces of the
piezoelectric element, and supporters to support the electromechanical
transducer, wherein the piezoelectric element is formed by directly
connecting the main faces of the two piezoelectric substrates, the
electromechanical transducer is formed by directly connecting the
piezoelectric substrates without using adhesive layers. As a result, an
acceleration sensor where flexible vibration generated due to acceleration
at its electromechanical transducer is not absorbed can be provided.
It is preferable in the first method that the main faces of the two
piezoelectric substrates are directly connected by heat-treating after the
piezoelectric substrates are hydrophilically treated and their main faces
are connected to each other. In this preferred embodiment, the main faces
of the two piezoelectric substrates are bonded firmly and directly at the
atomic level via oxygen or hydroxyl groups.
In the second method of this invention for producing an acceleration sensor
comprising an electromechanical transducer having piezoelectric element
formed by connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the opposite main faces of the
piezoelectric element, and supporters to support the electromechanical
transducer, wherein the piezoelectric element is formed by directly
connecting the supporters to the piezoelectric substrates composing the
piezoelectric element, the electromechanical transducer is directly
connected to the supporters without using adhesives, and, thus, the
variation of the supporting position of the electromechanical transducer
is reduced. As a result, an acceleration sensor whose resonance frequency
varies less can be obtained.
It is preferable in the second method that the supporters and the
piezoelectric substrates are directly connected by heat-treating after the
supporters and the piezoelectric substrates are hydrophilically treated
and connected to each other.
In the third method of this invention for producing an acceleration sensor
comprising an electromechanical transducer having piezoelectric element
formed by connecting opposing main faces of at least two piezoelectric
substrates and electrodes formed on the opposing main faces of the
piezoelectric element, supporters to support the electromechanical
transducer, and packages to contain the electromechanical transducer,
wherein the supporters are connected to the package directly and firmly,
acceleration generated on the mounting surface can be transferred to the
supporters via the packages without loss, and a high sensitive
acceleration sensor can be obtained.
It is preferable in the third method that the supporters are directly
connected to the packages by heat-treating after the supporters and the
packages are hydrophilically treated and connected to each other.
In the fourth method of this invention for producing an acceleration sensor
comprising an electromechanical transducer having piezoelectric element
formed by connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the opposite main faces of the
piezoelectric element, and packages to contain the electromechanical
transducer, wherein the piezoelectric substrates composing the
piezoelectric element are directly connected to the packages, the
electromechanical transducer can be connected firmly to the packages. As a
result, acceleration the packages receive can be transferred to the
electromechanical transducer without loss, and a highly sensitive
acceleration sensor can be obtained. In addition to that, the sections of
the acceleration sensor can be reduced since the packages function as the
supporters, and the production process can be simplified.
It is preferable in the fourth method that the piezoelectric substrates are
directly connected to the packages by heat-treating after the
piezoelectric substrates and the packages are hydrophilically treated and
connected to each other.
In the fifth method of this invention for producing an acceleration sensor
comprising an electromechanical transducer having a piezoelectric element
formed by connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the opposite main faces of the
piezoelectric element, supporters to support the electromechanical
transducer, and packages comprising at least two parts in order to contain
the electromechanical transducer, wherein the parts of the packages are
directly connected to each other, the parts composing the packages are
firmly connected to each other without using adhesives, and thus the heat
resistance properties of the connected faces are improved. As a result,
gases are not generated from the connected part even if solder reflow is
conducted, and each parts of the packages are air-tight sealed. Therefore,
a reliable acceleration sensor whose characteristics do not deteriorate
can be obtained.
It is preferable in the fifth method that the parts of the packages are
directly connected to each other by heat-treating after the parts of the
packages are hydrophilically treated and connected to each other.
A sixth method of this invention for producing an acceleration sensor
comprising an electromechanical transducer having a piezoelectric element
formed by connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the opposite main faces of the
piezoelectric element, and packages to contain the electromechanical
transducer comprising the following processes:
forming plural piezoelectric elements by directly connecting at least two
piezoelectric substrates on which plural cantilevers or both-ends clamped
structures are pattern-formed;
directly connecting the packages to the piezoelectric substrates, the
packages being formed with concavities to correspond to the piezoelectric
elements; and
separating the acceleration sensor into individual acceleration sensors
containing the piezoelectric elements. In this sixth method, the
electromechanical transducers are pattern-formed from the piezoelectric
substrates, and thus, the shape of the electromechanical transducer is
less varied. Since the electromechanical transducer and the supporters are
formed simultaneously, the supporting condition of the electromechanical
transducer is quite stable. Therefore, the cantilever or a beam clamped on
both ends does not vary much in length, and as a result, an acceleration
sensor which has very little variation in characteristics, such as
resonance frequency, can be obtained. In addition, the materials of the
electromechanical transducer, the supporters and the packages are the
same, so an extremely stable acceleration sensor can be provided without
the influence of distortion due to temperature. Another advantage of this
embodiment is superior quantity productivity, since a number of
acceleration sensors can be produced at a time on a substrate.
It is preferable in the sixth method of this invention that electrodes are
formed on the two opposite main faces of the piezoelectric element after
the formation of the piezoelectric element. In this preferred embodiment,
a mask can be located easily when the electrodes are formed, and also the
electrodes can be formed on the piezoelectric elements precisely, since
the piezoelectric elements are already formed. As a result, a precise
electromechanical transducer can be provided. In this case, it is also
preferable that conductive layer be formed on the piezoelectric substrate
when the electrodes are formed on the two opposite main faces of the
piezoelectric elements, so that the producing processes can be simplified.
It is further preferable in the sixth method of this invention that a
cantilever or a beam clamped on both ends is pattern-formed after the
electrodes are formed on the piezoelectric substrates. In this preferred
embodiment, an electromechanical transducer can be produced without
positioning the electrodes precisely. If a piezoelectric element is thin,
a short-circuit of the front and back electrodes can occur as the
electrodes are formed after the cantilever is pattern-formed. In this
preferred embodiment, however, such a problem can be prevented. It is
preferable in this embodiment that a conductive layer is formed on the
piezoelectric substrate when the electrodes are formed.
A seventh method of this invention for producing an acceleration sensor
comprising an electromechanical transducer having a piezoelectric element
formed by connecting two opposite main faces of at least two piezoelectric
substrates and electrodes formed on the opposite main faces of the
piezoelectric element, and packages to contain the electromechanical
transducer comprises the following processes:
forming plural piezoelectric elements by pattern-forming plural cantilevers
or a beam having both-ends structure after directly connecting at least
two piezoelectric substrates;
directly connecting the packages to the piezoelectric substrates, the
packages being formed with concavities to correspond to the piezoelectric
elements; and
separating the acceleration sensor into individual acceleration sensors
containing the piezoelectric elements.
It is preferable in the seventh method of this invention that electrodes
are formed on the two opposite main faces of the piezoelectric element
after the formation of the piezoelectric element. In this case, it is also
preferable that a conductive layer is formed on the piezoelectric
substrates when the electrodes are formed on the two opposite main faces
of the piezoelectric elements, so that the producing processes can be
simplified.
It is further preferable in the seventh method of this invention that a
cantilever or a beam clamped on both ends is pattern-formed after the
electrodes are formed on the piezoelectric substrates. It is preferable in
this embodiment that a conductive layer is formed on the piezoelectric
substrate when the electrodes are formed.
A shock detecting device of this invention comprises:
an acceleration sensor provided with an electromechanical transducer
comprising piezoelectric elements configured by connecting two opposite
main faces of at least two piezoelectric substrates and electrodes formed
on the main faces, and supporters to support the electromechanical
transducer;
an amplifier circuit which converts and amplifies signals from the
acceleration sensor;
a comparator circuit which compares the signals from the amplifier circuit
with standard signals;
a control circuit which controls the apparatus in which the acceleration
sensor is included; and
a storage device to store shock. The shock detecting device can measure
acceleration precisely since there is no variation in sensitivity of the
acceleration sensor or in its capacitance. Therefore, the shock detecting
device can detect and analyze shocks precisely by using the comparator
circuit depending on the standard values, while it can also instruct
recording of the detected shock and protect the apparatus from the shock
by analysis at its control circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing an electromechanical transducer used
for an acceleration sensor of the first embodiment of this invention.
FIGS. 2(a)-2(c) are flow diagrams which show the interface conditions of
the piezoelectric substrate in each step of direct connection in producing
an electromechanical transducer used for an acceleration sensor of the
first embodiment of this invention.
FIG. 3 is a perspective view showing another example of an
electromechanical transducer used for an acceleration sensor of the first
embodiment of this invention.
FIG. 4 is a perspective view showing a cantilever structure bimorph
electromechanical transducer of the first embodiment of this invention.
FIG. 5 is a cross-sectional view showing a cantilever structure bimorph
electromechanical transducer of the first embodiment of this invention.
FIG. 6 is an exploded perspective view showing an example of an
acceleration sensor of the first embodiment of this invention.
FIG. 7 is an explanatory view showing where flexible vibration is generated
in the cantilever structure bimorph electromechanical transducer of the
first embodiment of this invention.
FIG. 8 is a diagram showing the relationship between the crystal axis of
the piezoelectric substrate and the cut angle.
FIG. 9 is a graph showing the relationship between the cut angle of a
LiNbO.sub.3 substrate and its piezoelectric coefficients.
FIG. 10 is a graph showing frequency characteristics of the acceleration
sensor of the first embodiment of this invention.
FIG. 11 is a perspective view showing cut angles of the acceleration sensor
of the first embodiment of this invention.
FIG. 12 is a graph showing the relationship between the cut angles of a
quartz substrate and its piezoelectric coefficients.
FIG. 13 is a graph showing the relationship between the cut angles of a
quartz substrate and its piezoelectric coefficients.
FIG. 14 is a graph showing the relationship between the cut angles of a
quartz substrate and its piezoelectric coefficients.
FIG. 15 is a cross-sectional view showing an electromechanical transducer
used for an acceleration sensor of the second embodiment of this
invention.
FIG. 16 is an exploded perspective view of an example of an acceleration
sensor of the second embodiment of this invention.
FIG. 17 is a cross-sectional view of an example of an acceleration sensor
of the second embodiment of this invention.
FIG. 18 is an exploded perspective view of another example of an
acceleration sensor of the second embodiment of this invention.
FIGS. 19(a)-19(c) are flow charts showing a method for producing an
acceleration sensor of the third embodiment of this invention.
FIGS. 20(a)-20(c) are flow charts showing another method for producing an
acceleration sensor of the third embodiment of this invention.
FIG. 21 is an exploded perspective view of an example of an acceleration
sensor of the fourth embodiment of this invention.
FIG. 22 is an exploded perspective view of another example of an
acceleration sensor of the fourth embodiment of this invention.
FIG. 23 is a circuit diagram of a shock detecting device of the fifth
embodiment of this invention.
FIG. 24 is a diagram of an amplifier circuit of a shock detecting device of
the fifth embodiment of this invention.
FIG. 25 is a graph showing the relationship between output and acceleration
measured by using the shock detecting device of the fifth embodiment of
this invention.
FIG. 26 is a perspective view of a bimorph electromechanical transducer
according to the prior art.
FIG. 27 is a cross-sectional view of a cantilever structure bimorph
electromechanical transducer according to the prior art.
FIG. 28 is a cross-sectional view of a both-ends clamped structure bimorph
electromechanical transducer according to the prior art.
DETAILED DESCRIPTION OF THE INVENTION
The First Embodiment
FIG. 1 is a perspective view of an electromechanical transducer used for an
acceleration sensor of the first embodiment of this invention. As shown in
FIG. 1, a piezoelectric element 2 is formed by directly connecting two
opposite main faces of piezoelectric substrates (2a, 2b) which comprise
lithium niobate (LiNbO.sub.3). The rectangular piezoelectric substrates
are 50 .mu.m thick, 0.5 mm wide, 2 mm long. The piezoelectric substrates
(2a, 2b) are connected so that their polarization axes are directed
oppositely. Electrodes (3a, 3b) comprising 0.2 .mu.m thick of
chromium-gold are respectively formed on the two opposite main faces of
the piezoelectric element, forming a bimorph electromechanical transducer
1.
A method for producing such an electromechanical transducer is explained
below.
FIGS. 2(a)-2(c) are flow diagrams showing the interface conditions of the
piezoelectric substrates in each step of direct connection in producing an
electromechanical transducer used for an acceleration sensor of the first
embodiment of this invention. L.sub.1, L.sub.2 and L.sub.3 in FIGS.
2(a)-(c) indicate distance between the piezoelectric substrates. First,
both sides of the two LiNbO.sub.3 substrates (piezoelectric substrates
(2a, 2b)) were mirror-polished. Then, the piezoelectric substrates (2a,
2b) were hydrophilically treated by washing them in a mixture of ammonia,
hydrogen peroxide and water (ammonia water:hydrogen peroxide
water:water=1:1:6 in volume). As shown in FIG. 2(a), the surfaces of the
washed piezoelectric substrates (2a, 2b) are ended with hydroxyl groups
(--OH groups) and become hydrophilic (pre-connection condition).
In the next step, the two hydrophilic-treated piezoelectric substrates
LiNbO.sub.3 (2a, 2b) were connected so that the polarization axes are
oppositely directed (L.sub.1 >L.sub.2). As a result, the piezoelectric
substrates 2a and 2b are connected by attraction like --OH bonding or
hydrogen bonding due to dehydration.
The connected piezoelectric substrates (2a, 2b) were heat-treated at
450.degree. C. As shown in FIG. 2(c), atoms composing the piezoelectric
substrate 2a and those of the piezoelectric substrate 2b are covalently
bonded (L.sub.2 >L.sub.3) via oxygen (O), and thus the piezoelectric
substrates (2a, 2b) were firmly and directly connected at the atomic
level. In other words, a bonding having no adhesive layer (e.g. adhesives)
on the interface was obtained. In another embodiment, the atoms composing
the two piezoelectric substrates can be covalently bonded via hydroxyl
groups and the piezoelectric substrates can be firmly and directly
connected at the atomic level.
LiNbO.sub.3 's Curie-point is 1210.degree. C., and its characteristics will
deteriorate if its temperature history approaches that point. Therefore,
it is preferable that heat-treatment is conducted below the Curie-point.
A "direct connection" is a connection generated between the interfaces
directly without any adhesive layer like adhesive materials. For such a
direct connection, mirror-polished faces of materials to be connected are
surface-treated and contacted with each other. In general, heat-treatment
makes a connection by intermolecular force to a strong connection at the
atomic level, such as covalent bonding and ionic bonding.
Next, chromium-gold was evaporated by vacuum evaporation on the two
opposite main faces of the piezoelectric element 2 which is made up of
directly connected piezoelectric substrates LiNbO.sub.3 (2a, 2b), so that
the electrodes (3a, 3b) were formed (see FIG. 1). In the end, a bimorph
electromechanical transducer 1 was produced by cutting the piezoelectric
element 2 into strips of predetermined size by using a dicing saw.
As shown in FIG. 3, direct connection can be conducted via a buffer layer
46 comprising, for example, a silicon oxide thin film. More specifically,
a buffer layer 46 comprising 0.1 .mu.m thick of silicon oxide thin film is
formed on a main face of the piezoelectric substrate 2a. After the buffer
layer 46 and the other piezoelectric substrate 2b are hydrophilically
treated, they are laminated and heat-treated, so that the atoms composing
the piezoelectric substrate 2b and those composing the buffer layer 46 are
connected via oxygen or via hydroxyl groups. Even if faces to be connected
have waviness or irregularities, or even if foreign matter like
contaminants are stuck to the face, the buffer layer 46 will absorb them,
so direct connection can be conducted easily. When materials on which
oxygen or hydroxyl groups are hard to form by hydrophilic treatment,
direct connection can be conducted easily via the buffer layer 46. In this
embodiment, the buffer layer can be provided on one side or both sides of
the faces to be connected. Not only silicon oxide, but silicon nitride and
metallic silicide etc. can be used for the buffer layer.
FIG. 4 is a perspective view showing a cantilever structure bimorph
electromechanical transducer of the first embodiment of this invention,
and FIG. 5 is a cross-sectional view of the same electromechanical
transducer. As shown in FIGS. 4 and 5, a piezoelectric element 2 is formed
by directly connecting two opposite main faces of piezoelectric substrates
(2a, 2b) which comprise lithium niobate (LiNbO.sub.3). The rectangular
piezoelectric substrates are 50 .mu.m thick, 0.5 mm wide, and 2 mm long.
The piezoelectric substrates (2a, 2b) are connected so that their
polarization axes are directed oppositely. One end of the piezoelectric
element 2 is sandwiched by supporters (4a, 4b) comprising LiNbO.sub.3 and
fixed. The piezoelectric element 2 is directly connected to the supporters
(4a, 4b). In this embodiment, the piezoelectric element 2 and the
supporters (4a, 4b) can be also directly connected via a buffer layer
comprising, for example, a silicon oxide thin film. Electrodes (3a, 3b)
comprising 0.2 .mu.m thick of chromium-gold are respectively formed on the
two opposite main faces of the piezoelectric element and these electrodes
are formed successively on the supporters (4a, 4b), and thus, a cantilever
structure bimorph electromechanical transducer 1 is formed.
FIG. 6 is an exploded perspective view of an example of an acceleration
sensor of the first embodiment of this invention. As shown in FIG. 6, a
cantilever structure bimorph electromechanical transducer 1 is set in a
package 10b comprising LiNbO.sub.3. A depression is formed in the center
of the package 10b by etching etc. In other words, the supporters (4a, 4b)
of the bimorph electromechanical transducer 1 are fixed in the package 10b
by conductive pastes (5a, 5b; 5a is not seen) in the condition that the
bimorph electromechanical transducer 1 excepting the supporters (4a, 4b)
is maintained above the depression. Another LiNbO.sub.3 package 10a is
overwrapped in the package 10b and adhered to it. The package 10a is also
provided with a depression (not shown) at its center. Therefore, the whole
of the bimorph electromechanical transducer 1 is covered with the packages
(10a, 10b). Conductive layers (7a, 7b) comprising silver-palladium are
formed inside the package 10b. One end of each conductive layer (7a, or
7b) is electrically connected to the electrode (3a, or 3b) of the bimorph
electromechanical transducer 1 via the conductive paste (5a, or 5b). Outer
electrodes (9a, 9b) comprising nickel are formed on both ends of the
packages (10a, 10b). The other ends of the conductive layers (7a, 7b) are
electrically connected to the outer electrodes (9a, 9b). In other words,
the electrodes (3a, 3b) of the bimorph electromechanical transducer 1 are
electrically connected to the outer electrodes (9a, 9b) respectively via
the conductive pastes (5a, 5b) and via the conductive layers (7a, 7b). As
a result, electric charge generated in the bimorph electromechanical
transducer 1 can be run out. An acceleration sensor 100 has the
configuration mentioned above.
If something attached to the acceleration sensor 100 shown in FIG. 6 is
accelerated, a force proportional to the acceleration is transferred to
the electromechanical transducer 1 via the packages (10a, 10b) and the
supporters (4a, 4b). If vertical acceleration (arrow in FIG. 4) is
generated, the electromechanical transducer 1 bends vertically and thus,
flexible vibration occurs (see FIG. 7). When the electromechanical
transducer 1 comprising the piezoelectric substrates (2a, 2b) bends
downward (solid line in FIG. 7), the piezoelectric substrate 2a is
distorted (becomes long) due to tensile force, since the piezoelectric
substrate 2a is located above the central axis of the electromechanical
transducer 1. On the other hand, the piezoelectric substrate 2b which is
located below the central axis of the electromechanical transducer 1 is
distorted (shrinks) due to compressive force.
In a conventional electromechanical transducer produced by adhering
piezoelectric ceramic substrates, adhesives softer than the piezoelectric
substrates exist between the substrates. Therefore, when the
electromechanical transducer bends vertically, the flexible vibration is
absorbed by the adhesive materials, and as a result, stress generated on
the piezoelectric substrates is decreased, and so is the electromotive
force generated on the piezoelectric substrates.
On the other hand, the electromechanical transducer 1 of this invention is
produced by directly connecting the piezoelectric substrates (2a, 2b), so
there is no adhesive layer like adhesives between the piezoelectric
substrates 2a and 2b. Namely, when flexible vibration occurs on the
electromechanical transducer 1 because of acceleration, nothing will
absorb this flexible vibration. Therefore, stress generates at the
piezoelectric substrates (2a, 2b), and great electromotive force can be
obtained. As a result, a highly sensitive acceleration sensor can be
provided. In addition to that, variation of resonance frequency of the
electromechanical transducer 1 can be remarkably reduced and sensitivity
thus increases, since the piezoelectric substrates (2a, 2b) connection
becomes uniform. Furthermore, the vibration characteristics of the
electromechanical transducer 1 do not vary depending on temperature, since
adhesive layers do not exist between the piezoelectric substrates (2a,
2b).
The supporters (4a, 4b) are firmly connected to the electromechanical
transducer 1 at the atomic level, and thus the acceleration given to the
matter having the packages (10a, 10b) is transferred to the
electromechanical transducer 1 without loss.
Though electric charge corresponding to compressive stress and tensile
stress is generated on the upper and lower faces of the piezoelectric
substrates (2a, 2b) comprising LiNbO.sub.3, electric charge of the same
polarity generates on the two piezoelectric substrates even if the stress
generated on the substrates differs from the compressive stress and
tensile stress, since the piezoelectric substrates (2a, 2b) are connected
to each other so that the direction of their polarization axes is the
opposite. Namely, electromotive force occurs on the piezoelectric
substrates (2a, 2b) in the same direction (see FIG. 7). Therefore, signals
reflecting the acceleration can be obtained from the electrodes (3a, 3b)
formed on both faces of the electromechanical transducer 1. When the
thickness direction of the LiNbO.sub.3 substrate is predetermined to be
the Y' axis direction and the longitudinal direction is the Z' direction,
the compressive force and the tensile force act in the Z' axis direction
while the electric charge is generated in the Y' direction. In this case,
the quantity of the generated charge is largely depends on piezoelectric
coefficient d.sub.23 '. The value of the piezoelectric coefficient
d.sub.23 ' greatly varies depending on the directions of the Y' and Z'
axes to the crystal axis. Namely, the sensitivity of the acceleration
sensor greatly varies depending on the directions of the Y' and Z' axes.
The best sensitive acceleration sensor can be obtained when the Y' and Z'
axes are set properly and cut angle is selected in order to have the
largest absolute value of the piezoelectric coefficient d.sub.23 '.
FIG. 8 is a diagram showing the relationship between the crystal axis of
the piezoelectric substrate and a cut angle. In FIG. 8, the X axis, Y axis
and Z axis indicate the crystal axis direction of LiNbO.sub.3, while the
X' axis (=X axis), Y' axis and Z' axis indicate the perpendicular axis
where the Y axis is rotated by .theta. degrees with respect to the X axis.
Namely, the X' axis (=X axis), Y' axis and Z' axis indicate the cut
directions of the LiNbO.sub.3 substrate. When the direction of each axis
is predetermined as shown in FIG. 8, the piezoelectric coefficient
d.sub.12 ' has a great influence on the sensitivity of the acceleration
sensor when the thickness direction is the X' axis direction, and the
longitudinal direction is the Y' direction. When the thickness direction
of the substrate is predetermined to be the X' axis direction and the
longitudinal direction is the Z' axis direction, piezoelectric coefficient
d.sub.13 ' gives a great influence on the sensitivity of the acceleration
sensor. When the thickness direction of the substrate is predetermined to
be the Y' axis direction and the longitudinal direction is the X' axis
direction, piezoelectric coefficient d.sub.21 ' has a great influence on
the sensitivity of the acceleration sensor. When the thickness direction
of the substrate is predetermined to be the Y' axis direction and the
longitudinal direction is the Z' axis direction, piezoelectric coefficient
d.sub.23 ' has a great influence on the sensitivity of the acceleration
sensor. When the thickness direction of the substrate is predetermined to
be the Z' axis direction and the longitudinal direction is the X' axis
direction, piezoelectric coefficient d.sub.31 ' has a great influence on
the sensitivity of the acceleration sensor. When the thickness direction
of the substrate is predetermined to be the Z' axis direction and the
longitudinal direction is the Y' axis direction, piezoelectric coefficient
d.sub.32 ' has a great influence on the sensitivity of the acceleration
sensor.
FIG. 9 is a graph showing the relationship between the cut angle of a
LiNbO.sub.3 substrate and its piezoelectric coefficient. As shown in FIG.
9, the piezoelectric coefficient d.sub.23 ' has its maximum value when the
cut angle is 140.degree.. Experimental results for an electromechanical
transducer having various cut angles are shown in the following Table 1
and in FIG. 10.
TABLE 1
______________________________________
Cut angle Piezoelectric coefficient
Sensitivity
(.degree. ) d.sub.23 ' (C/N)
(mV/G)
______________________________________
Y-cut 135.degree.
Z' direction
-2.99 .times. 10.sup.-11
6.1
Y-cut 135.degree.
X direction
1.40 .times. 10.sup.-11
3.7
Y-cut 140.degree.
Z' direction
-3.06 .times. 10.sup.-11
6.4
Y-cut 140.degree.
X direction
1.53 .times. 10.sup.-11
3.8
Y-cut 165.degree.
Y direction
1.98 .times. 10.sup.-11
4.6
Z-cut 0.degree.
X direction
0.0 No sensitivity
Z-cut 0.degree.
Y direction
0.0 No sensitivity
______________________________________
As shown in the above Table 1, the electromechanical transducer generated
the maximum electric charge when the Y-cut 140.degree. substrate having
the biggest piezoelectric coefficient was used and the Z' axis was the
longitudinal direction. FIG. 11 shows the influence of the cut angle. As
shown in FIG. 11, the best sensitivity was obtained when the Y axis was
rotated by 140.degree. about the X angle, an electrode was formed on a
face perpendicular to the Y' axis, and the Z' axis was the longitudinal
direction.
The crystal structure of LiNbO.sub.3 is trigonal 3m class, and has a
symmetric structure with the Z axis three times. Thus, there are several
cut angles having the same piezoelectric coefficients. For example, the
piezoelectric coefficient d.sub.32 ' of cut angles 50.degree. and of
230.degree. are the same as d.sub.23 ' of cut angles of 140.degree. and of
320 . This depends on the symmetry of the crystals.
If the cut angle is close to the best cut angle where the sensitivity is
the maximum, the piezoelectric coefficient is less influenced by the cut
angle, thus substantially the same sensitivity can be obtained even if the
cut angle is not optimized precisely. Several problems will be found if
the cut angle is optimized precisely. For example, the process should be
accurate, and the cost will rise because the processes will be complicated
to reduce variation, and the unit price also rises due to bad yield.
Though the best cut angle of the piezoelectric coefficient d.sub.23 ' is
140.degree. as shown in FIG. 9, the piezoelectric coefficient is 90-100%
of its maximum value when the cut angle is 129-152.degree., and problems
due to sensitivity deterioration do not arise. When the thickness
direction of the LiNbO.sub.3 substrate is Y' axis and the longitudinal
direction is X axis, the sensitivity of the acceleration sensor depends on
the piezoelectric coefficient d.sub.21 '. As shown in FIG. 9, the cut
angle dependency of the piezoelectric coefficient d.sub.21 ' is smaller
than that of d.sub.23 '. In this case, if the cut angle is in the range of
.+-.26.degree., the piezoelectric coefficients become 90-100% of the
maximum value, so problems due to sensitivity deterioration do not arise.
Therefore, a highly sensitive acceleration sensor can be produced without
precise cut angle, and the process cost can be lowered. If the cut angles
of two piezoelectric substrates differ from each other, the electric
charge caused by the pyroelectric effect of the piezoelectric element
cannot be cancelled out, so the difference of cut angles should be
decreased. The range of the cut angle difference between the two
piezoelectric substrates composing the electromechanical transducer can be
1.degree. or less, since the piezoelectric coefficients do not greatly
depend on the cut angles.
The length, thickness and width of the electromechanical transducer 1 is
determined based on the frequency range of the acceleration to be
measured. If the frequency of the measured acceleration is close to the
resonance frequency of the electromechanical transducer, the sensitivity
of the acceleration sensor is improved. When a cantilever structure
electromechanical transducer 1 is prepared by directly connecting two
LiNbO.sub.3 substrates of 50 .mu.m thick and by determining the length
from its top to its supporter to be 2 mm, the resonance frequency is 20
kHz. As clear from the measured relationship of sensitivity to
acceleration frequency shown in FIG. 10, when the acceleration frequency
is 10 kHz or more, the sensitivity is improved since the frequency is
close to the resonance frequency of the electromechanical transducer 1.
The resonance frequency should be separated sufficiently from the
measurement frequency range so that the sensitivity of the acceleration
sensor is not greatly influenced by the frequency. For this purpose, for
example, the electromechanical transducer 1 should be set so that the
resonance frequency becomes twice as the maximum measurement frequency. As
mentioned above, the resonance frequency of the electromechanical
transducer 1 is determined depending on its length and thickness. In a
conventional acceleration sensor using a piezoelectric ceramic, the
electromechanical transducer is supported via adhesives. It is difficult
to control the amount of the adhesive, and thus the length of the
electromechanical transducer varies quite substantially. For example,
extra adhesive will make the electromechanical transducer 1 shorter. As a
result, in an acceleration sensor using a conventional piezoelectric
ceramic, the resonance frequency of the electromechanical transducer
varies, and thus several problems are found, such as the sensitivity
variation in the high-frequency region. In addition, the frequencies which
can be measured will change respectively. In this embodiment, the
supporters (4a, 4b) are directly connected to the electromechanical
transducer 1, so the length variation of the electromechanical transducer
1 becomes remarkably small. Therefore, the resonance frequency of the
electromechanical transducer 1 is extremely small, and thus, the
sensitivity in the high-frequency region of the respective acceleration
sensor and the measurement frequency range vary much less.
LiNbO.sub.3 as a piezoelectric material has a single crystal structure, and
thus the variations of its piezoelectric coefficients, dielectric
constant, elastic coefficient are very small. In a piezoelectric ceramic,
these material constants generally vary by about 20%. In the acceleration
sensor produced by directly connecting LiNbO.sub.3 substrates, the
variations of sensitivity and of resonance frequency were reduced to 5% or
less.
Furthermore, the piezoelectric ceramic deteriorates with time, and thus it
is unstable. Therefore, in an acceleration sensor using piezoelectric
ceramic, the sensitivity changes by 10-15% over time. On the other hand,
the acceleration sensor produced by directly connecting LiNbO.sub.3
substrates is so stable that it deteriorates 2% at most over time.
In addition to LiNbO.sub.3, single crystal piezoelectric materials which
can be directly connected are used for the piezoelectric substrates (2a,
2b), for example, lithium tantalate (LiTaO.sub.3), quartz, and
Langasite-type piezoelectric crystals. Langasite-type piezoelectric
crystals include La.sub.3 Ga.sub.5 SiO.sub.14, La.sub.3 Ga.sub.5.5
Nb.sub.0.5 O.sub.14, and La.sub.3 Ga.sub.5.5 Ta.sub.0.5 O.sub.14. The
crystal structure of LiTaO.sub.3 is of a trigonal system 3m class like
LiNbO.sub.3, and the best cut angle is identical to that of LiNbO.sub.3.
The crystal structures of quartz and Langasite-type piezoelectric crystals
are of trigonal system 32 classes. FIGS. 12-14 show the relationship
between the cut angle of a quartz substrate having 32 classes crystal
structure and the piezoelectric coefficients. FIG. 12 shows the
relationship between the cut angle rotated about the X axis and the
piezoelectric coefficients. FIG. 13 shows the relationship between the cut
angle rotated about the Y axis and the piezoelectric coefficients. FIG. 14
shows the relationship between the cut angle rotated about the Z axis and
the piezoelectric coefficients. The relationship between the cut angle of
Langasite-type piezoelectric crystal substrates and piezoelectric
coefficients is same as this.
Table 2 shows the best cut angles of single crystal piezoelectric materials
of 32 classes crystal structures, and those of single crystal
piezoelectric materials of 3m classes structures. The best cut angles are
obtained from FIGS. 12-14.
TABLE 2
______________________________________
Piezoelectric
Crystal Best cut angle
constant having
structure
Material (Eulerian angle)
polar value
______________________________________
3m trigonal
Lithium niobate
140,0,0 d.sub.23 '
system Lithium tantalate
0,0,0 d.sub.21 '
32 trigonal
Quartz 70,0,0 d.sub.13 '
system Langasite-type
0,0,0 d.sub.12 '
piezoelectric
0,0,90 d.sub.21 '
crystals
______________________________________
Note: The cut angles rotated about the Z axis by 120.degree. and those
rotated about the X axis and Y axis by 180.degree. are included.
In Table 2, the three numerals of Eulerian angles indicate rotation angles
about the X, Y, and Z axes respectively. The maximum piezoelectric
coefficients are also indicated in Table 2. When a piezoelectric material
having 32 classes crystal structure is rotated by 70.degree. about the X
axis, the maximum piezoelectric coefficient is d.sub.13 '. It is clear
from this fact that the best sensitivity will be obtained when a
piezoelectric material having 32 classes crystal structure is rotated by
70.degree. about the X axis, the piezoelectric substrate is cut out
perpendicularly to the rotated X' axis (=X axis) direction, and the
longitudinal direction of the piezoelectric substrate is set in the Z'
axis direction. When the Eulerian angle is (0, 0, 0), the maximum
piezoelectric coefficient is d.sub.12 '. In other words, the sensitivity
is the best if the piezoelectric substrate is cut out perpendicular to the
rotated X' axis (=X axis) direction and the longitudinal direction of the
piezoelectric substrate is set in the Y' axis direction. When the Eulerian
angle is (0, 0, 90), the maximum piezoelectric coefficient is d.sub.21 '.
In other words, the sensitivity is the best if the piezoelectric substrate
is cut out perpendicular to the rotated Y' axis (=Y axis) direction and
the longitudinal direction of the piezoelectric substrate is set in the X'
axis direction.
In a piezoelectric substrate having 32 classes crystal structure (e.g.
quartz, Langasite-type piezoelectric crystals), the piezoelectric
coefficients become 90-100% of the maximum value when d.sub.13 ' is from
+52.degree. to +86.degree., d.sub.12 ' is within .+-.26, or d.sub.32 ' is
from +52.degree. to +68.degree., so problems due to sensitivity
deterioration will not arise. Therefore, the process cost can be lowered
since a highly sensitive acceleration sensor can be produced even if the
cut angle is not precise.
In this embodiment, the electromechanical transducer 1 is prepared by a
strong direct connection of the piezoelectric substrates (2a, 2b) without
using adhesive layers. Therefore, a highly sensitive acceleration sensor
can be obtained without characteristic variation or decrease of vibration.
In addition, the electromechanical transducer 1 can be located precisely
since the electromechanical transducer 1 is directly connected to the
supporters (4a, 4b) without using adhesives. As a result, a small
acceleration sensor can be obtained. The length and supporting conditions
of the cantilever do not vary, and thus the characteristics of the
acceleration sensor are remarkably stable. The electromechanical
transducer 1 can be located precisely since the electromechanical
transducer 1 is directly connected to the supporters (4a, 4b) without
using adhesives, so acceleration is transferred to the electromechanical
transducer 1 without loss.
In this embodiment, chromium-gold is used for the electrodes (3a, 3b).
Gold, chromium, silver, and alloy also can be used as the electrode
material.
In this embodiment, LiNbO.sub.3 is used for the packages (10a, 10b).
LiTaO.sub.3, quartz, glass, plastics and ceramics like alumina also can be
used for the material.
In this embodiment, the electromechanical transducer 1 has a cantilever
structure. It is also possible that the electromechanical transducer 1 can
be made to have a both-end clamped structure by directly connecting its
ends to supporters. The center of the electromechanical transducer 1 also
can be directly connected to the supporters.
The Second Embodiment
FIG. 15 is a cross-sectional view of an electromechanical transducer used
for an acceleration sensor of the second embodiment of this invention. As
shown in FIG. 15, a piezoelectric element 2 is formed by directly
connecting two opposite main faces of piezoelectric substrates (2a, 2b)
which comprise lithium niobate (LiNbO.sub.3). The rectangular
piezoelectric substrates are 50 .mu.m thick, 0.5 mm wide, 2 mm long. The
piezoelectric substrates (2a, 2b) are connected so that their polarization
axes are directed oppositely. One end of the piezoelectric element 2 is
fixed in a condition sandwiched by supporters (4a, 4b) comprising
LiNbO.sub.3. The piezoelectric element 2 is directly connected to the
supporters (4a, 4b). The opposite main faces of the piezoelectric element
2 have portions which are not sandwiched by the supporters. Chromium-gold
electrodes (3a, 3b) of 0.2 .mu.m thick are formed at these non-sandwiched
portions. Therefore, a cantilever structure bimorph structure
electromechanical transducer 1 is formed.
FIG. 16 is an exploded perspective view of an example of an acceleration
sensor of the second embodiment of this invention, and FIG. 17 is a
cross-sectional view of FIG. 16. As shown in FIGS. 16 and 17, the
cantilever structure electromechanical transducer 1 of FIG. 15 is set in a
package 10c comprising LiNbO.sub.3 and the supporters (4a, 4b) are
directly connected to the inside of the other end face of the package 10c.
The upper face, one end face, and one side of the package 10c are open.
The supporters (4a, 4b) and the package 10c also can be directly connected
via a buffer layer comprising silicon oxide thin film. Ends of conductive
layers (7c, 7d) are connected to the electrodes (3a, 3b) along the
supporters (4a, 4b) and the package 10c. The other ends of the conductive
layers (7c, 7d) are exposed at the end of the package 10c. A package 10d
having the same shape as package 10c and comprising LiNbO.sub.3 is
connected to the package 10c. Outer electrodes (9c, 9d) comprising nickel
are formed on the outsides of the packages (10c, 10d). The outer
electrodes (9c, 9d) are electrically connected to the conductive layers
(7c, 7d) respectively. In other words, the electrodes (3a. 3b) are
electrically connected to the outer electrodes (9c, 9d) respectively via
the conductive layers (7c, 7d). Therefore, the electric charge generated
in the electromechanical transducer 1 can conduct out. An acceleration
sensor 102 is formed as mentioned above.
In the acceleration sensor 102 shown in FIG. 17, the electromechanical
transducer 1 vibrates vertically and flexible vibration occurs if
acceleration is applied vertically. Once flexible vibration occurs,
certain faces of the piezoelectric substrates (2a, 2b) are
stretch-distorted while the other faces are shrink-distorted. The
piezoelectric substrates (2a, 2b) are connected so that the directions of
the polarization axes are opposed. Namely, electromotive force is
generated on the piezoelectric substrates (2a, 2b) in the same direction.
As a result, signals which reflect the acceleration can be obtained from
the electrodes (3a, 3b) formed on the both faces of the electromechanical
transducer 1.
FIG. 18 is an exploded perspective view of another example of an
acceleration sensor of the second embodiment of this invention. As shown
in FIG. 18, a piezoelectric element 2 is formed by directly connecting
opposite main faces of piezoelectric substrates (2a, 2b) which comprise
lithium niobate (LiNbO.sub.3). The rectangular piezoelectric substrates
are 50 .mu.m thick, 0.5 mm wide, 2 mm long. The piezoelectric substrates
(2a, 2b) are connected so that their polarization axes are directed
oppositely. Chromium-gold electrodes (3a, 3b) of 0.2 .mu.m thick are
formed on the two opposite main faces. Therefore, a cantilever structure
bimorph electromechanical transducer 1 is formed. One end of the
electromechanical transducer 1 is sandwiched between packages (10e, 10f)
comprising LiNbO.sub.3 which are opened vertically and separated into two
parts. And thus, the electromechanical transducer 1 is supported. The
packages (10e, 10f) are directly connected to the electromechanical
transducer 1, and the same packages are directly connected to each other.
The direct connection of the packages to the electromechanical transducer,
or the direct connection between the packages can be conducted via buffer
layers comprising a thin silicon oxide layer. Packages (10g, 10h) are
fixed with adhesive to the upper and lower faces of the packages (10e.
10f). As a result, the electromechanical transducer 1 is sealed in the
package. Outer electrodes (9e, 9f) comprising nickel are formed on the end
faces of the packages, and this outer electrodes (9e, 9f) are electrically
connected to the electrodes (3a, 3b) via conductive layers (not shown).
Therefore, electric charge generated in the electromechanical transducer 1
can conduct out. Depressions are formed inside the packages (10g, 10h) so
that the electromechanical transducer is not contacted when the
electromechanical transducer 1 bends. An acceleration sensor 103 has the
configuration mentioned above.
The length, thickness and width of the electromechanical transducer 1 is
determined based on the frequency range of the acceleration to be
measured. As the frequency of the measured acceleration gets close to the
resonance frequency of the electromechanical transducer 1, the sensitivity
of the acceleration sensor is improved. The resonance frequency should be
separated sufficiently from the measurement frequency range so that the
sensitivity of the acceleration sensor is not greatly influenced by the
frequency. For this purpose, for example, the electromechanical transducer
1 should be set so that the resonance frequency is twice the maximum
measurement frequency.
LiNbO.sub.3 (a piezoelectric material) has a single crystal structure, and
thus the variations of its piezoelectric coefficients, dielectric
constant, elastic coefficient are very small. In a piezoelectric ceramic,
these material constants generally vary by about 20%. In the acceleration
sensor produced by directly connecting LiNbO.sub.3 substrates, the
variations of sensitivity and of resonance frequency were reduced to 5% or
less.
Furthermore, the piezoelectric ceramic deteriorates over time, and thus it
is unstable. Therefore, in an acceleration sensor using a piezoelectric
ceramic, the sensitivity changes by 10-15% over time. On the other hand,
the acceleration sensor produced by directly connecting LiNbO.sub.3
substrates is so stable that it deteriorates 2% at most over time.
In this embodiment, the electromechanical transducer 1 is prepared by a
strong direct connection of the piezoelectric substrates (2a, 2b) without
using adhesive layers. Therefore, a highly sensitive acceleration sensor
can be obtained without characteristic variation or decrease of vibration.
In addition, the electromechanical transducer 1 can be located precisely
since the electromechanical transducer 1 is directly connected to the
supporters (4a, 4b) without using adhesives. As a result, a small
acceleration sensor can be obtained. The length and supporting conditions
of the cantilever do not vary, and thus the characteristics of the
acceleration sensor are remarkably stable. The electromechanical
transducer 1 can be located precisely since the electromechanical
transducer 1 is directly connected to the supporters (4a, 4b) without
using adhesives, so acceleration is transferred to the electromechanical
transducer 1 without loss. In addition to that, the supporters (4a, 4b)
are directly connected to the packages without using adhesives. As a
result, the supporters (4a, 4b) are firmly connected to the packages to
which acceleration from the mounting face is directly transferred, and
thus acceleration can be transferred without loss to the electromechanical
transducer 1.
In this embodiment, LiNbO.sub.3 is used for the piezoelectric substrates
(2a, 2b). LiTaO.sub.3 and quartz also can be used for the material.
In this embodiment, gold, chromium, silver, or alloy also can be used for
the electrodes (3a, 3b), in addition to chromium-gold.
In this embodiment, glass, ceramics, or resin also can be used for the
packages, in addition to LiNbO.sub.3.
In this embodiment, direct connection is used for fixing of the supporters
(4a, 4b) of the electromechanical transducer 1 to inside the packages or
for fixing the packages. Fixing with adhesives may also be useful.
In this embodiment, the electromechanical transducer 1 has a cantilever
structure. It is also possible that the electromechanical transducer 1 can
be made to have a both-end clamped structure by directly connecting its
ends to supporters. The center of the electromechanical transducer 1 also
can be directly connected to the supporters.
The Third Embodiment
The cantilever structure acceleration sensor shown in FIGS. 16-18 and
method for producing the same will be explained below.
FIGS. 19(a)-19(c) and FIGS. 20(a)-19(c) are flow charts showing a method
for producing an acceleration sensor of the third embodiment of this
invention.
In the first step shown in FIG. 19(a), cantilevers (11a, 11b) were formed
by sandblasting where LiNbO.sub.3 substrates were used for piezoelectric
substrates (12a, 12b) and photoresist patterns are masking reagents.
Next, as shown in FIG. 19(b), the piezoelectric substrate (12a) formed with
the cantilever (11a) and piezoelectric substrate (12b) formed with the
cantilever (11b) are directly connected to each other. The direct
connection was conducted by connecting the hydrophilic-treated two
piezoelectric substrates (12a, 12b) so that the polarization axes are
opposite, and heat-treating the substrates. The direct connection can also
be conducted via a buffer layer comprising silicon oxide layers.
In the next step shown in FIG. 19(c), electrodes (13a, 13b) were formed on
the both sides of the cantilever by evaporating chromium-gold, using
vacuum evaporation. Therefore, a cantilever structure bimorph
electromechanical transducer 15 was obtained. The electromechanical
transducer 15 is supported with the apertures of the piezoelectric
substrates as supporters. A conductive layer 14a was formed on the
piezoelectric substrate 12a. The electrode 13a and the conductive layer
14a were on the same side and they conduct to each other. This conductive
layer 14a is used to conduct electric charge generated on the electrode
13a to the outer electrode 20a (see FIGS. 20(b) and (c)). A conductive
layer 14b was formed on the piezoelectric substrate 12b on which the
electrode 13b was formed to conduct to the opposite side (right hand of
the figure) of the aperture on the piezoelectric substrate 12b and the
cantilever. The producing processes can be simplified by simultaneously
forming the conductive layer 14a or 14b with the electrode 13a, 13b.
Next, packages (16a, 16b) were formed by forming a concavity 17 on another
LiNbO.sub.3 substrate by sandblasting, where photoresist patterns were
used for masking reagents (see FIG. 20(a)). Simultaneously, a penetrating
hole 18 was formed in the packages (16a, 16b) in order to be electrically
connected to the conductive layers (14a, 14b) on the piezoelectric
substrates (12a, 12b).
As shown in FIG. 20(b), the piezoelectric substrates (12a, 12b) formed with
the electromechanical transducer 15 and the packages (16a, 16b) were
connected to each other by direct connection. As a result, the
electromechanical transducer 15 was sealed inside the packages (16a, 16b).
The direct connection between the piezoelectric substrates (12a, 12b) and
the packages (16a, 16b) also can be carried out via a buffer layer
comprising silicon oxide thin layer. The cantilever was pattern-formed
simultaneously with the supporters from the LiNbO.sub.3 substrate, and
thus the supporters also might bend. Actually, however, the cantilever was
still supported firmly by the packages (16a, 16b) and another LiNbO.sub.3
substrate which functioned as the supporters. Conductive layers
(chromium-gold) (14a, 14b) were formed on the connecting portions between
the piezoelectric substrates of LiNbO.sub.3 (12a, 12b) and the packages of
LiNbO.sub.3 (16a, 16b). Therefore, it was difficult to directly connect
the piezoelectric substrates to the packages. However, the connection can
be strong if the connecting area between the piezoelectric substrates and
the packages are sufficient in size compared to the size of the conductive
layers (14a, 14b). Then, through-fall conductive portions (19a, 19b) were
formed by pouring conductive paste into the penetrating hole 18 on the
packages (16a, 16b) in order to electrically connect to the conductive
layers (14a, 14b), and firing the structure thus formed. Moreover,
silver-palladium was printed on the upper faces of the packages (16a, 16b)
in order to be conductive to the through-fall portions (19a, 19b), and
thus outer electrodes (20a, 20b) were formed. As a result, the electrodes
(13a, 13b) on the electromechanical transducer 15 and the outer electrodes
(20a, 20b) were electrically connected to each other.
As shown in FIG. 20(c), the substrate was cut off into acceleration sensors
104 by using a dicing saw. The electromechanical transducer 15 has a
cantilever, and it is firmly connected to the packages (16a, 16b).
In the acceleration sensor 104 shown in FIG. 20(c), the electromechanical
transducer 15 vibrates vertically and flexible vibration occurs if
acceleration is applied vertically. Once flexible vibration occurs,
certain faces of the piezoelectric substrates (12a, 12b) are
stretch-distorted while the other faces are shrink-distorted. The
piezoelectric substrates (12a, 12b) are connected to each other so that
the direction polarization axes are opposed. Namely, electromotive force
is generated on the piezoelectric substrates (12a, 12b) in the same
direction. As a result, signals which reflect the acceleration can be
obtained from the electrodes (13a, 13b) formed on the both faces of the
electromechanical transducer 15.
The length, thickness and width of the electromechanical transducer 15 is
determined based on the frequency range of the acceleration to be
measured. As the frequency of the measured acceleration becomes close to
the resonance frequency of the electromechanical transducer 15, the
sensitivity of the acceleration sensor 104 is improved. The resonance
frequency should be separated sufficiently from the measurement frequency
range so that the sensitivity of the acceleration sensor 104 is not
greatly influenced by the frequency. For this purpose, for example, the
electromechanical transducer 15 should be set so that the resonance
frequency is twice the maximum measurement frequency.
In this embodiment, the electromechanical transducer 15 is prepared by a
strong direct connection of the piezoelectric substrates (12a, 12b)
without using adhesive layers such as adhesives. Therefore, a highly
sensitive acceleration sensor 104 can be obtained without characteristic
variation or decrease of vibration. The shape variation of the
electromechanical transducer 15 can be controlled since the
electromechanical transducer 15 is pattern-formed from a piezoelectric
substrate. In addition, the supporting condition of the electromechanical
transducer 15 is comparatively stable since the electromechanical
transducer 15 and the supporters are formed simultaneously. Thus, the
cantilever does not greatly vary in length and the characteristics of the
acceleration sensor are remarkably stable. The materials of the
electromechanical transducer 15, supporters and the packages (16a, 16b)
are identical, thus, the influences of temperature, including distortion,
can be reduced. Therefore, a small acceleration sensor excellent in
stability can be provided. The acceleration sensor is also superior in
quantity production since numbers of acceleration sensors are produced
simultaneously from a substrate.
In this embodiment, the material for the piezoelectric substrates (12a,
12b) is not limited to LiNbO.sub.3, but other materials like LiTaO.sub.3
and quartz can also be used.
In this embodiment, the material for packages (16a, 16b) is not limited to
LiNbO.sub.3, but other materials like LiTaO.sub.3, quartz, silicon, and
glass can also be used. It is preferable that the material is identical to
that of the piezoelectric substrates (12a, 12b) composing the
electromechanical transducer 15. It is further preferable that the
material has a coefficient of thermal expansion similar to that of the
piezoelectric substrates (12a, 12b) composing the electromechanical
transducer 15.
In this embodiment, the material of the electrodes (13a, 13b) is not
limited to chromium-gold, but other materials like gold, chromium, silver
and alloy can also be used.
In this embodiment, the material for the through-fall conductive portions
(19a, 19b) is not limited to conductive paste, but other materials like
solder and silver solder can also be used.
In this embodiment, a bimorph electromechanical transducer 15 is produced
by forming cantilevers (11a, 11b) on the piezoelectric substrates (12a,
12b) and then directly connecting these parts. The order, however is not
limited to this. It is also possible that the cantilevers (11a, 11b) are
formed after the two piezoelectric substrates (12a, 12b) are directly
connected.
In this embodiment, the electrodes (13a, 13b) are formed after the
formation of the cantilevers (11a, 11b). The order, however, is not
limited to this. It is also possible to form the cantilevers (11a, 11b)
after the electrodes (13a, 14b) are formed.
In this embodiment, sandblasting is used for processing the cantilevers
(11a, 11b) on the piezoelectric substrates (12a, 12b), the concavity 17
and the penetrating hole 18 on the packages (16a, 16b). The method,
however, is not limited to sandblasting. Other methods like dry etching,
wet etching, razor processing, ion beam processing, dicing, mechanical
processing including wire saw cutting, water jet processing and plasma
processing, can also be used.
The method for forming electrodes (13a, 13b) is not limited to vacuum
evaporation, but other methods like sputtering, vapor-phase film forming
methods including chemical vapor deposit (CVD), plating and printing may
be used.
Though in this embodiment the outer electrodes (20a, 20b) are formed on the
upper face of the packages (16a, 16b), the electrodes can be provided on
the sides of the packages or extending from the side to the top of the
packages.
The method for connecting the conductive layers (14a, 14b) to the outer
electrodes (20a, 20b) is not limited to providing the penetrating hole 18
on the packages (16a, 16b), but it is also possible to cut the packages
(16a, 16b) partially for the same purpose.
In case the piezoelectric substrates of LiNbO.sub.3 (12a, 12b) are not
connected strongly enough to the packages of LiNbO.sub.3 (16a, 16b) due to
the existence of the conductive layers (14a, 14b), the connection strength
can be improved by forming a buffer layer of silicon oxide film on the
connecting face and carrying out connection via the buffer layer.
The Fourth Embodiment
FIG. 21 is an exploded perspective view of an example of an acceleration
sensor of the fourth embodiment of this invention. As shown in FIG. 21, a
bimorph electromechanical transducer 21 is connected to a package 27b
comprising LiNbO.sub.3, and the both ends of the electromechanical
transducer are supported (both ends clamped structure). The bimorph
structure electromechanical transducer 21 is produced by directly
connecting the piezoelectric substrates of LiNbO.sub.3 (22a, 22b) to each
other. Another package 27a comprising LiNbO.sub.3 is directly connected to
the package 27b. The direct connection between the packages (27a, 27b) can
also be carried out via a buffer layer comprising, for example, a thin
silicon oxide film. Outer electrodes (26a, 26b) are respectively connected
(26b is not shown) to the outside of the packages (27a, 27b). These outer
electrodes (26a, 26b) are electrically connected to the electrodes (23a,
23b) of the electromechanical transducer 21. Therefore, the electric
charge generated in the electrodes (23a, 23b) on the electromechanical
transducer 21 can conduct out. An acceleration sensor 105 has the
configuration mentioned above.
The both ends clamped structure acceleration sensor 105 can be produced by
the same method as the third embodiment. As shown in FIG. 22, a bimorph
electromechanical element is produced by forming a both ends clamped
structure on the two piezoelectric substrates (22a, 22b) and directly
connecting the piezoelectric substrates to each other. A bimorph
electromechanical transducer 21 is prepared by forming electrodes (23a,
23b) on the both ends clamped structure. Next, conductive layers (24a,
24b) are formed (24b is not shown) to connect the outer electrodes to the
electrodes (23a, 23b). This both ends clamped structure electromechanical
transducer 21 can be mass-produced on a substrate as in the third
embodiment. A LiNbO.sub.3 substrate is prepared by forming a concavity and
a penetrating hole in the process described in the third embodiment.
Packages are formed by directly connecting the LiNbO.sub.3 substrate on
the piezoelectric substrates (22a, 22b) formed with the electromechanical
transducer 21. An acceleration sensor is completed by forming the other
elements, such as the outer electrodes.
The resonance frequency of the acceleration sensor having both-ends clamped
structure is higher than that of an electromechanical transducer having a
cantilever if the length and size are common, thus, acceleration in a
higher frequency region can be measured.
The length, thickness and width of the electromechanical transducer 21 are
determined based on the frequency range of the acceleration to be
measured. As the frequency of the measured acceleration gets close to the
resonance frequency of the electromechanical transducer 21, the
sensitivity of the acceleration sensor 105 is improved. The resonance
frequency should be separated sufficiently from the measurement frequency
range so that the sensitivity of the acceleration sensor 105 is not
greatly influenced by the frequency. For this purpose, for example, the
electromechanical transducer 21 should be set so that the resonance
frequency is twice the maximum measurement frequency.
In this embodiment, the electromechanical transducer 21 is prepared by a
strong direct connection of the piezoelectric substrates (22a, 22b)
without using adhesive layers. Therefore, a highly sensitive acceleration
sensor can be obtained without characteristic variation or decrease of
vibration. In addition, the electromechanical transducer 21 can be located
precisely and the length and the supporting condition of the both ends
clamped structure do not vary since the electromechanical transducer 21 is
directly connected to the packages (27a, 27b). As a result, a small
acceleration sensor which can measure at a higher frequency range will be
obtained. The acceleration sensor is stable and its characteristic
variation is extremely small. In this embodiment, the packages (27a, 27b)
are firmly connected directly without using adhesives, and thus the heat
resistance of the connecting faces is improved. Therefore, the connecting
parts do not generate gases even if solder reflow is conducted, and thus
every part composing the packages (27a, 27b) is air-tight sealed. As a
result, a reliable acceleration sensor whose characteristics do not
deteriorate can be obtained.
The material for the piezoelectric substrates (22a, 22b) is not limited to
LiNbO.sub.3, but other materials like LiTaO.sub.3 and quartz are also
used.
In this embodiment, the material for packages (27a, 27b) is not limited to
LiNbO.sub.3, but other materials like LiTaO.sub.3, quartz, silicon, and
glass can also be used. It is preferable that the material is identical to
that of the piezoelectric substrates (22a, 22b) composing the
electromechanical transducer 21. It is further preferable that the
material has a coefficient of thermal expansion similar to that of the
piezoelectric substrates (22a, 22b) composing the electromechanical
transducer 21.
The Fifth Embodiment
FIG. 23 is a circuit diagram of a shock detecting device of the fifth
embodiment of this invention. As shown in FIG. 23, a shock detecting
device of this invention comprises an acceleration sensor 40, an amplifier
circuit 41 which transfers and amplifies signals outputted from the
acceleration sensor 40, a comparator circuit 42 which compares the signals
outputted from the amplifier circuit 41 with standard signals, a storage
device 43 which stores shocks, a display device 44 which displays the
shock, and a control circuit 45 which controls the comparator circuit 42,
the storage device 43, and the display device 44. A control device of an
apparatus to which this shock detecting device is set can be used
partially for the control circuit 45. In this embodiment, the acceleration
sensor of the first embodiment (FIG. 6) is used for the acceleration
sensor 40.
FIG. 24 is a diagram of the amplifier circuit. As shown in FIG. 24, the
acceleration sensor 40 is connected to the gate of a field effect
transistor (FET) in parallel to the resistance R.sub.1, so signals
outputted from the acceleration sensor 40 are inputted to the FET gate.
The cut off frequency of the low frequency side in the frequency
characteristics of this circuit depends on the resistance R.sub.1 and the
capacitance C.sub.1 of the acceleration sensor 40, and it is expressed by
the equation: f.sub.c =1/2.pi.R.sub.1 C.sub.1. Therefore, the output is
decreased if the frequency is lower than f.sub.c. If the capacitance
C.sub.1 of the acceleration sensor 40 varies considerably, the cut off
frequency f.sub.c also varies, and the lowest measurable frequency
changes. In an acceleration sensor using a piezoelectric ceramic, the
capacitance variation is about 20%, and the cut off frequency varies by
the same degree. On the other hand, in the acceleration sensor 40 produced
by directly connecting the piezoelectric single crystal substrates of this
invention, the variation of the capacitance C.sub.1 is only 7% or less, so
the cut-off frequency f.sub.c is also stable, and as a result, a stable
output can be obtained to low frequency acceleration. The signals
outputted from the acceleration sensor 40 are transferred at the amplifier
circuit 41 and amplified. The signals outputted from the amplifier circuit
41 are inputted to the comparator circuit 42. The comparator circuit 42
decides whether the signals from the amplifier circuit are larger than the
standard signals or not.
The following is an example where a shock of 10G or more is recognized,
recorded, and displayed. FIG. 25 is a graph showing the relationship
between output voltage and acceleration where the output of the
acceleration sensor described in the first embodiment is measured by using
the circuits shown in FIG. 24. As shown in FIG. 25, when a 10G
acceleration is applied, the signal outputted from the amplifier circuit
41 connected to the acceleration sensor 40 is 64 mV. In this case, 64 mV
of standard signal is inputted from the control circuit 45 to the
comparator circuit 42. This standard signal and the output signals from
the amplifier circuit 41 are compared at the comparator circuit 42, and if
the output signals from the amplifier circuit 41 are larger, output
signals generated at the comparator circuit 42 are sent back to the
control circuit 45. In this way, the control circuit 45 recognizes a shock
larger than the standard 10G. The control circuit 45 can order the storage
device 43 to store the shock larger than the standard 10G. At the same
time, the level and the date and time of the shock can also be recorded.
The control circuit 45 orders the display device 44 to display the shock
larger than the standard 10G, the level and the date and time of the
shock, and thus the display device 44 displays the information. The
control circuit 45 can order the whole apparatus on which this shock
detecting device is installed to avoid malfunction and destruction caused
by the shock. In case this shock detecting device is installed in a hard
disc device, the device can order it to stop writing information from the
head on the disc, and to retract the head in order to avoid the situation
in which the head bumps against the disc and is damaged. The shock
detecting device installed in a portable phone can order self-decision on
its failure.
The acceleration sensor (FIG. 6) indicated in the first embodiment is used
for the shock detecting device of this invention, and thus there is no
variation of sensitivity of the acceleration sensor or of the capacitance.
Therefore, a shock detecting device provided by this invention realizes
precise measurement of acceleration and provides accurate decisions of
shocks comparing to a standard value, by using its comparator circuit. The
shock detecting device having the above-mentioned configuration can decide
and order some operations including detection and recording of the shock
and protection of the apparatus from the shock, by using its control
circuit.
This invention may be embodied in other forms without departing from the
spirit or essential characteristics thereof. The embodiments disclosed in
this application are to be considered in all respects as illustrative and
not limitative, the scope for the invention is indicated by the appended
claims rather than by the foregoing description, and all changes which
come within the meaning and range of equivalency of the claims are
intended to be embraced therein.
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